Investment casting is a type of precision casting for metals, also known as the lost wax process. A pattern model identical to the desired workpiece to be produced is initially made from wax or other materials. Multiple patterns produced by wax injection may be joined to other wax pieces to create a so called “wax assembly”. The “wax assembly” goes through a sequence of shell-build operations to encase the pattern in mold material. Molten metal is then poured into the fired and pre-heated mold to produce the rough casting of the desired work-piece. Tight dimensional control throughout the process is essential to yield a so called ‘near-net-shape’ casting requiring minimal machining.
Typically, the shell-build process requires two types of slurry so called “prime” and “backup”. The prime slurry, used for the first, and/or second coats, consists of finer particle-size refractory powder, typically −325 mesh refractory powder, and aqueous colloidal sol with organic polymer. Prime slurries have high solids content and need to have rheological characteristics to produce a uniform coating to replicate all of the pattern detail in the mold and casting. Typically, the “prime slurry” contains surfactants to allow wetting of the slurry on the pattern and antifoam emulsion to reduce surface tension and minimize entrapped air and facilitate efficient mixing of the slurry raw materials. The “backup” slurry consists of coarser powder, typically −200 and or −120 mesh refractory powder, aqueous colloidal sol with organic polymer at lower solids content and is used for all coats except the first or second coats. After each slurry and stucco combination (referred to as a coating) is applied, a drying operation is performed in a temperature and humidity controlled environment to prepare for the next coating until all coats are applied. Use of organic polymer in slurries, introduced to the investment casting industry in the mid 1980's, provides essential plasticity and toughness to the coatings during the drying and “dewax” operations. Prior to the use of polymer, colloidal silica alone (which forms a water insoluble bond), provided the strength throughout all the shell-build operations; dipping, drying, “dewax”, firing, and casting. Historically, colloidal silica has played a key role for producing ceramic shell molds in the investment casting industry.
The so called “dewax” operation is performed by steam autoclave or flash-fire to remove the wax and pattern material. Flash-fire, introduced in the 1990's by Pacific Kiln, performs both dewax and mold firing simultaneously. The resultant mold from either method must yield a clean mold cavity free of all residue with a smooth hard surface. Firing is typically performed in the 1,800 to 2,000 deg F. range. After preheating of the mold molten metal is then poured into the mold cavity and solidified. Finally, the raw casting is obtained by removal of the ceramic mold material. Shell removal can be accomplished by impacting the cast runners with a hammer or by waterblast.
Known methods for slurry formulation use colloidal silica (a stable dispersion of silicon oxide particles), alumina, zirconia or yttria sol with particles less than 300 nm in size in a continuous aqueous medium. Aqueous colloidal silica, nominally 30% solid and balance water, is used in a variety of grades; small particle, large particle, and polymer enhanced. Colloidal silica has been the preferred binder for precision investment casting since ethyl silicate was phased out in the 1980's. Colloidal silica does have some favorable characteristics. Colloidal silica forms a permanent bond to itself that is ideal for dipping, drying, steam autoclave, and strength at high temperatures. Colloidal silica particles sinter and bind the refractory particles together. This provides the needed mechanical strength for dipping, drying, dewax, and casting operations. As a result, colloidal silica is the binder of choice in the majority of precision investment casting foundries.
However, aerospace manufacturers are designing more and more components based on light weight and reactive alloys. Casting manufacturers need better mold materials that are less reactive to cast alloys like titanium aluminide and the expanding demand for single-crystal components. Therefore, the ability to make molds that have reduced silica, or silica-free and less reactive, can be fired to higher temperatures (1900 to 2,200 deg. F.) are very desirable characteristics that colloidal silica bonded molds have a hard time delivering. Demands dictated by advanced alloys include: higher mold firing temperatures, thermal conditions in casting, and the availability of silica-free binders. For example, jet engine turbine blades rotating at up to 5,000 rpm at 1,000 degrees C. for up to 15 hours must perform flawlessly.
Silica-free mold face-coats would also be a huge benefit for casting titanium alloys. It is common knowledge that silica binder causes a reaction with elements like titanium, hafnium, yttrium, and aluminum that can lead to oxide inclusions or a case-hardened surface.Ti(liq.)+SiO2(soild)→TiO2(solid)+Si
The ‘case’, hardened and brittle surface layer, developed during casting must be removed by a special high-temperature chemical soaking operation. Additionally, oxide inclusions can become flaws that initiate premature failure. So, manufacturer suppliers of components realize customer tolerance for imperfections in aerospace industries is basically non-existent. For this reason, precision investment casting processes need the highest quality raw materials to produce flawless products.
While molds made from colloidal-silica-bonded slurry can produce quality cast articles, there are many drawbacks and consequences. Initially, bulky transport is required for the liquid. The environment must be controlled to prevent freezing and degradation. The stability of colloidal silica has many factors including pH, particle size, silica concentration, and storage temperature. Sols should be stored at 5-35° C. (40-95° F.). If the sol is subjected to freezing conditions, it can lose its stability and precipitate. Highly elevated temperatures may accelerate the growth of micro-organisms and/or decrease the long-term stability of the silica sol. pH ranges are very important to the stability of the sol. For example, if the pH of the “prime” slurry approaches 9.2, the binder starts to gel and should not be used in that state to manufacture molds. “Ostwald ripening” leads to agglomeration of the very small silica particle dispersions and the surface area will slowly decrease. The latter results in a critical reduction in strength of colloidal silica bonded molds. That is why companies have dedicated laboratories and technicians to regularly confirm the quality of the colloidal silica binder in the production slurries. Silica concentration is also very important for stability. The more concentrated a sol, the more likely the particles will be forced together and allowed to aggregate. Stability generally determines the shelf life of a sol. Checking sol stability involves performing an ‘oven gel test’ which requires 24 hours to perform. Either production has to be suspended during that period, or production continues under a cloud of suspicion. As evidenced above, even with transportation and storage capabilities, shelf life monitoring, the gelling of the binder creates an atmosphere of doubt and risk associated with colloidal silica-bonded molds used to produce precision castings. Furthermore, even if skilled technicians determine the binder in a slurry has gelled it is unknown how much product is at risk because of the 24-hour period needed to test the binder by the ‘oven gel test’.
Furthermore, the use of mild steel or iron with colloidal silica is discouraged because the iron will discolor the product and destabilize the dispersion. Lastly, cleaning colloidal silica can involve using a caustic soda solution of 4-5% caustic soda (NaOH), agitating for 2-5 hours at 50-60° C.
Regarding alternatives to molds produced with silica sols it is common knowledge that non-silica sol bonded molds must be dewaxed by flash-firing as they break down in a steam autoclave dewax. Colloidal zirconia, yttria, and alumina are common presently commercially available options for low reactive prime coats. Since those products are nearly 100% of those oxides they require very high temperatures to develop sinter-bonding with those products. So, if the backup slurries are silica-bonded they would be over-fired if the mold was fired at 2100 F to develop the needed sintering that the non-silica sols require. Furthermore, when producing large molds with silica sols, the maximum firing temperature may need to be limited to prevent mold distortion associated with softening of the colloidal silica.
Presently aqueous colloidal silica is used in some way throughout the investment casting industry. Furthermore, transport of colloidal silica must be done under temperature controlled conditions and during winter months stored in a heated warehouse. A way to produce investment casting molds without transporting water and storing water would significantly reduce the energy and cost associated with colloidal silica or any aqueous oxide sol binder for that matter. Furthermore, in the aerospace investment casting sector, such as single crystal casting operations, the use of colloidal silica has limitations; 1) molds produced with colloidal silica, fired to high temperature, are frequently too strong leading to defects and special shell removal operations, and 2) The use of desired elements such as yttrium, titanium, hafnium, and aluminum is limited due to reaction with colloidal silica. So, reduced silica or silica-free binder is a significant advantage in the aerospace investment casting sector. Therefore, producing molds that are simultaneously less reactive and easier to remove has significant advantage.
The reference “Effect of Mold Material and Binder on Metal-Mold Interfacial Reaction for Investment Casting of Titanium Alloys by Kim, teaches the negative effects of using colloidal silica binders for titanium. Kim clearly shows increased reaction and increased hardness by reaction with the silica from colloidal binders.
What is needed is a dry, reduced silica, powder material which, when combined with water instead of aqueous colloidal silica sol, forms a refractory investment slurry that produces molds for castings having accurate dimensions, that avoid cracks and settling away, and maintains structural integrity during steam autoclave dewax, while reducing transportation, storage, and preparation costs associated with aqueous colloidal silica binder. The dry powder, and slurries produced therefrom, needs to fit within the present operations and processes without causing major disruption that would result in inconvenience and major equipment changes by precision casting manufacturers.